The present invention relates to the technical field of recovering an etch bath from an etching process in which printed boards coated with copper are etched with the alkaline etch bath and then rinsed with water. The recovery of copper from such an etch bath as well as the recovery of the etch bath itself are known per se, but the invention relates to the recovery of copper in a manner that is completely new in this connection and very advantageous.
Today, electronics are primarily composed of components which are soldered onto printed boards. The manufacture of these printed boards has increased rapidly. Advanced electronics, such as computers, require very complicated printed boards and the manufacture thereof has resulted in the emergeing of a special industry.
A printed board consists of a plate made of a base material, which is often a plastic material, e.g. epoxy. This plate is coated with a thin (e.g. about 15-20 xcexcm) copper layer on both sides.
The manufacture of these printed boards may, for instance, be carried out in the following manner. According to a predetermined pattern, holes of different size are drilled into the plates so that ducts are formed between the two sides and so that holes are formed, in which components can be mounted. After that, all surfaces are coated with a copper layer which has a thickness of about 2-4 xcexcm and which is applied in a chemical manner instead of electrochemically. This is necessary since the walls of the holes consist of a material (e.g. epoxy) on which it is not possible to use electrochemical plating. As a result, an integral and continuous copper surface is obtained, which can subsequently be coated by means of electrochemical plating.
The next step in the manufacture of printed boards is to make the desired conductive pattern. To this end, the surfaces which are to be free from copper are coated with a non-conductive protective film, and the boards are fed into a so-called electroplating line. This line comprises a number of different baths, into which the boards are dipped.
In one of these baths, the exposed copper surfaces are coated with additional copper to provide the desired thickness of conductors, after which the same surfaces are coated with a tin layer, which will serve as a protection during the subsequent etching.
After that, the etching of the printed boards in question takes places, which is usually carried out by means of alkaline solutions. Preferably, they are ammoniacal and contain free ammonia as well as one or more ammonium salts (e.g. NH4Cl, NH4HCO3). When etching, copper is removed from the surfaces of the printed board, on which the copper layer is exposed to the solution. Gradually, the copper content of the bath rises to a level, on which the speed of etching drastically decreases and the bath is no longer usable. Usually, the upper limit of the copper content is about 150-170 g/l, depending on the composition of the bath. After the etching, the printed boards are rinsed with water to be clean.
The boards are then passed on to be stripped of tin and coated with varnish, wax, etc.
A method for regenerating the etch bath in and process of this kind is known from EP-B1-0005415. This method may be directly connected to the etching process and it involves in short the regeneration of the etch bath for renewed use in an etching process, copper being removed from the etch bath by extraction. The method also allows treatment of the rinsing water so that it may subsequently be let out in an environmentally suitable manner.
The copper which is extracted from the etch bath and preferably also from the rinsing water is recovered as metallic copper by electrolysis.
The present invention relates to a new and, in this connection, extremely advantageous process for recovering copper from an alkaline etch bath of the above-mentioned kind. According to the invention, it has thus surprisingly been found that it is possible not only to recover copper from the etch bath and preferably also from the rinsing water in an alternative new manner but also to reuse recovered copper-in the plating process. Generally, the process according to the invention makes it possible to directly reuse a copper-containing solution which is obtained after the extraction from the alkaline etch bath in the plating of the printed boards, if a flow is diverted from the copper-containing acid solution which is obtained from the re-extraction step and the balance of the copper content is adjusted between this flow and the acid solution which is withdrawn for recovering copper, e.g. metallic copper by electrolysis. As a result, the need for cost-demanding electrolysis for recovering metallic copper is reduced, at the same time as the need for additives of expensive anode copper is reduced or eliminated in the plating. In addition, it is possible to directly and in the same place use the recovered copper which previously could only be sold at a price of about xc2xc of the price of said anode copper while achieving a plating quality which is at least equally good or even better than the quality obtained in plating according to the plating technique which was previously commercially used.
Moreover, one condition of successful plating, i.e. with a satisfactory evenness of surface and satisfactory coating in holes and the like, in connection with commercial methods previously used for electroplating has been the addition of a number of different chemicals. Besides the fact that these chemicals to a certain extent have not been compatible with reagents which are used in the extraction, and therefore must be removed according to prior-art technique, which has complicated the process and made it more expensive, it has thus been found that the use of these chemicals may be reduced or completely eliminated in the process according to the present invention, if the plating is carried out under special conditions, in particular pulse plating. According to the invention, it has thus been found that at least equally good results as according to prior-art technique may be achieved by this new method according to the invention, which results in considerable advantages in terms of both costs and environment. Besides the fact that the process is simpler and cheaper, the process according to the invention also allows a completely closed or continuous process, in which also the plating step may be included in the prior-art etching and recovery process.
Further advantages of the invention will appear from the description below.
In this connection, it can be added that pulse plating of printed boards is known per se from Processing of Advanced Materials (1994) 9, pp 148-154, but this publication does not in any way disclose or even suggest that such plating could be integrated in a process of the kind involved in the present invention and even less so under the same circumstances and with the same results as in the case of the present invention.
Thus, the process according to the invention is a process for recovering copper from an alkaline, preferably ammoniacal, etch bath from an etching process in which printed boards electroplated with copper are etched with the alkaline etch bath and then rinsed with water, a part of the copper content of the alkaline etch bath being removed by extraction with an organic solution containing a reagent, which forms with copper a complex compound, which is extracted by the organic solution, the alkaline etch bath being recirculated to renewed etching, the copper-containing organic solution being contacted, in a re-extraction step, with an aqueous solution of acid, preferably sulphuric acid, so that copper passes from the organic solution to the aqueous solution, and the organic solution being recirculated from the re-extraction step to renewed extraction. The process according to the invention is characterised by the steps of passing the copper-containing acid solution obtained from the re-extraction step to a copper recovery operation, preferably for producing metallic copper by electrolysis, diverting a flow from the copper-containing acid solution before the operation for recovering copper from the same and adjusting the copper content of said flow so that it will be lower than the copper content of the acid solution which is used in the copper recovery operation, and re-circulating said flow having an adjusted copper content to the operation for electroplating printed boards for use therein.
According to a preferred embodiment of the process. copper is also removed from the rinsing water from the etching process by extraction with an organic solution containing a reagent, which forms with copper a complex compound which is extracted by the organic solution.
One embodiment of the last-mentioned process involves using the same organic solution for the rinsing water as for the alkaline etch bath and first removing copper from the alkaline etch bath, then contacting the thus obtained copper-containing organic solution with the rinsing water and subsequently subjecting the organic solution to said re-extraction.
Another embodiment of said process involves using the same organic solution for the rinsing water as for the alkaline etch bath and first removing copper from the rinsing water, then contacting the thus obtained copper-containing organic solution with the alkaline etch bath and subsequently subjecting the organic solution to said re-extraction.
A particularly preferred embodiment of the process according to the invention implies that it is carried out as a closed process, in which the plated printed board is etched with said alkaline etch bath and the acid solution from the plating is used for said re-extraction step. Most preferably, also the acid solution from the copper recovery operation is recirculated to the re-extraction step or steps.
According to the invention, it has been found that excellent results are achieved if the flow is adjusted so that the ratio of copper consent of said flow:copper content of said acid solution is  greater than 0.3:1, preferably  greater than 0.5:1. According to a particularly preferred embodiment of the process according to the invention, the flow is adjusted so that said ratio is in the range of 0.60:1-0.95:1, most preferably 0.75:1-0.95:1.
As suggested above, a particularly advantageous embodiment of the process implies that the plating is carried out in the form of pulse plating. In this connection, pulse plating refers to plating with pole reversal and/or pulses varying for current intensity. This can be achieved with suitable rectifier equipment. The varying pulses and/or the pole reversal may be regularly or irregularly wave-shaped, suitably square. As concerns this technique as such, information may be retrieved from the above-mentioned publication on pulse plating in general, but according to the invention it has been found that advantageous results are achieved under special conditions.
These conditions may be summarised as follows.
The pulse plating is preferably carried out with a pulse length of the wave-shaped pulses in the range of 1-500 ms, preferably 10-50 ms. In the case of these pulses, it is possible to have pulse lengths varying within the indicated ranges or use substantially the same, or exactly the same, pulse length for pulse peaks as for pulse bases, i.e. the period of time during which the wave-shaped, preferably square, pulses have maximum and minimum current intensity, respectively, alternatively maximum current intensity for +xe2x88x92voltage and xe2x88x92voltage, respectively.
Preferably, the period of time during which the printed board acts as cathode in the pulse plating is adjusted to a value in the range of 1-200 s, in particular 10-100 s.
The period of time during which the printed board acts as anode in the pulse plating is preferably adjusted to a value in the range of 0.1-20 s, in particular 1-10 s.
The maximum current intensity during the period of time when the printed board acts as cathode in the pulse plating is suitably 10 A/dm2, preferably 5 A/dm2 and most preferably 3 A/dm2.
The maximum current intensity during the period of time when the printed board acts as anode in the pulse plating is preferably 40 A/dm2, more preferably 10 A/dm2 and most preferably 5 A/dm2.
Generally, the average current intensity when the printed board acts as anode is greater than the average current intensity when the printed board acts as cathode.
According to the invention, it has furthermore been found that excellent results are achieved if the copper content of the flow which is recirculated to the plating is adjusted to a value in the range of 5-100 g/l, preferably 10-50 g/l, more preferably 15-30 g/l and most preferably 20-25 g/l.
In the case of such an adjustment, it has been found that an adjustment of the copper content of the flow which is recirculated to the plating functions extremely well by the addition of acid from the re-extraction step. As a rule, it is preferably the same order of magnitude of the copper content of the acid solution which is used in the copper recovery operation, use being made, however, of the important inventive adjustment of the ratio of the copper content of said flow to the copper content of said acid solution. The copper recovery operation is suitably, but not necessarily, an operation for recovering metallic copper by electrolysis. Details about this type of electrolysis as such can be retrieved from prior-art technique.
Generally, the acid content, or the content of anion from the acid which is used in the re-extraction and which is preferably sulphate, when the preferred acid is sulphuric acid, is adjusted to a value in the range of 25-500 g/l, preferably 50-200 g/l. This is the case of both the acid solution for the copper recovery operation and the flow for the plating, which moreover suitably have substantially, or exactly, the same acid content.
Another variant of the process according to the invention is represented by the case in which the polarity of the plating is reversed after the plating of copper, so that previously plated copper can be used as a buffer store of copper if the amount of copper obtained from the extraction is less than the amount required for the plating.
Another great advantage of the invention is, as already suggested above, that the new process according to the invention allows pulse plating without any additives of the type previously used in non-pulse plating of printed boards. Besides the fact that this, of course, involves great economic gains, it has also contributed to making it possible to integrate the plating in the described advantageous manner in a copper recovery process of the defined type.
Among other factors having an influence on the plating, mention can be made of the content of alkaline substance originating from the etch bath and the content of organic material originating from the extraction. According to the invention, it has thus been found that excellent results may be achieved in the process according to the invention if the content of said alkaline substance and/or the content of said organic material in the flow recirculated to the pulse plating is reduced, and preferably eliminated, before plating.
This reduction or these reductions may be carried out at different stages in the process according to the invention, but a particularly preferred embodiment is represented by the case where the reduction(xe2x88x92s) in question is/are carried out by means of one or more separate water washing steps in connection with the equipment which is used for the extraction. Such water washing steps are preferably integrated before the last step of the extraction process.
The reduction(xe2x88x92s) in question may also be carried out by means of one or more filters, preferably filters of charcoal and/or ultrafilter type. Such filters may also be placed at various locations in the process, but they are advantageously positioned in the separate loop which is represented by the flow which is recirculated to the plating.
Another advantageous embodiment of the process according to the invention is represented by the case in which colloidal copper is removed before the plating, preferably by means of one or more filters, in particular ultrafilters, from the flow which is recirculated to the pulse plating.
As will be described in more detail below in connection with the embodiments shown in the drawings, mixer-settler devices have been used in prior-art extraction. According to the invention, it has, however, been found that particularly favourable results in the plating may be achieved if these mixer-settler devices are wholly or partly replaced by one or more extractors. Such extractors are, of course, known per se, but they have been found to yield advantageous results in connection with the invention. By extractor is essentially meant a device in which the above-mentioned settler in the mixer-settler device is replaced by a centrifuge or some other separation means to which energy is supplied from the outside, whereas a settler is essentially based on the use of the force of gravity.